PTC means a characteristic that electrical resistance rapidly increases at a relatively narrow temperature range due to increase of temperature. PTC composites have such PTC characteristics and they are generally used in a circuit protection element, which limits current of a circuit when the circuits such as a heater, a positive-characterized thermistor, an ignition sensor, a battery or the like are short-circuited. The circuit protection element makes the circuit recovered when the cause of the short circuit is removed.
As another example employing the PTC composites, there is a PTC element in which at least two electrodes are electrically connected to such composites. Such a PTC element is used as an element for preventing over current or overheat, which acts for self-control of temperature, as described above.
Over-current prevention mechanism using the PTC element is as follows. At an ambient temperature, the PTC composite has a sufficiently low resistance, so ensuring current flow through a circuit. However, if a high current passes through the circuit due to, for example, a short circuit, this high current causes Joule heat generated in the PTC element, which increases temperature and therefore raises resistance of the element by the PTC characteristics. This resistance blocks current flow through the element, so protecting the circuit. It is generally referred as a current limiting property.
Such PTC element, or PTC composite, needs to have a current limiting property, which can repeatedly work even under high voltage. Also, improvement of the current limiting property comes from sufficient decrease of an initial resistance of the PTC element as well as endowment of the effective PTC characteristics.
There are developed many kinds of PTC composites. As an example, a PTC composite made by adding univalent or trivalent metal oxide to BaTiO3 is already well known. However, such composite has a problem that it allows current flow less than 1 msec because it shows NTC (Negative Temperature Coefficient) characteristics right after the PTC characteristics is manifested.
As an alternation, there has been developed a PTC composite, which is made by dispersing electrical conductive particles such as carbon black, carbon fiber, carbon graphite or metal particles to an organic polymer such as polyethylene, polypropylene or ethylene-acrylic acid copolymer. Such PTC composite is generally made by blending a necessary amount of electrical conductive particles into at least one resin, used as an organic polymer.
Reference can be made for example to U.S. Pat. No. 3,243,753, U.S. Pat. No. 3,823,217, U.S. Pat. No. 3,950,604, U.S. Pat. No. 4,188,276, U.S. Pat. No. 4,272,471, U.S. Pat. No. 4,414,301, U.S. Pat. No. 4,425,397, U.S. Pat. No. 4,426,339, U.S. Pat. No. 4,427,877, U.S. Pat. No. 4,429,216, U.S. Pat. No. 4,442,139 and so on.
In addition, Korean Patent Publication No. 99-63872 discloses a technique of grafting conductive particulate fillers into maleic anhydride grafted polyethylene in order to make a PTC composite. This PTC composite may show great adhesion to a metal electrode with a soft surface, recover its initial or lower resistance after repeated cycling (that is, changing from a low resistance state to a high resistance state and then returning), and extend a period of a tripped state.
However, any one among them does not show a technique to control a switching temperature and a trip time by adding polyethylene, on which a maleic anhydride is grafted, into crystalline polymer compounds.